Clutch and Flywheel Guide for the Subaru WRX and STI

What the Clutch Holds

Clutch capacity is torque at the flywheel, averaged across a crank revolution. A 250 hp high-rpm engine and a 250 hp turbo diesel need very different clutches, because the diesel makes about three times the torque. Horsepower can match while the torque, and the clutch, do not.

The first shopping mistake is buying too much clutch. Extra holding power arrives with chatter, noise, a heavier pedal, shorter facing life, a higher price, or a harder hit into the gears. Take the compromises you actually want. Leave the rest.

That order of facings is the general rise in holding force: organic, Kevlar, ceramic, feramic, carbon, then sintered iron. A maker’s own recipe can move a material around inside that list. Stepping to a more aggressive facing can add anywhere from about 10% to about 60% torque capacity. That spread is the recipe, so one brand’s organic is a different part from another brand’s organic.

Friction Materials

Most makers rate a clutch to the point where it slips, on average torque per crank revolution, rather than to a torque it can hold all day. A disc that is only even with your peak torque starts slipping sooner than the printed number suggests. One maker rates on slipping friction, so the printed capacity sits under the torque where the disc actually breaks away. Exedy publishes one rating for road course and another for drag. Two brands’ torque numbers are different tests. Compare inside one brand, and leave a margin over the torque you actually make.

Organic facing is a fiber blend with metal strands, the original-equipment style. It covers a wide temperature range and will take hard use. Repeated abuse overheats it. The resin that binds it melts when the disc is very hot and acts like a lubricant, so the clutch slips instead of sending the full hit into the gear teeth. That slip is why the stock organic disc is the kindest clutch to a WRX or STI gearbox. After it cools, it comes back close to the grip it had.

Kevlar holds up to hard use better and engages in a similar way. In stop-and-go traffic it can glaze, then slip until a hard use wears the glaze off. Overheat it and it stays cooked. Ceramic and carbon both take high temperature and both wear the flywheel faster in traffic. Carbon is slightly more durable, and slightly easier on the flywheel, than the other aggressive facings. Feramic mixes graphite with sintered iron. That mix is built for friction, torque capacity, and a smoother engagement.

A full carbon-on-carbon assembly is the carbon clutch that lasts. Carbon facing on a metal plate slips sooner and wears out faster, especially once it is hot, and it needs replacement more often. A true carbon-on-carbon disc can be refreshed only so far. It does not rebuild the way a metal plate does, with a new set of facings.

Sintered iron is the harsh end. It behaves like a switch, and it has to run on a flywheel surface made for it. A puck, also called a paddle, is the small-contact construction that usually carries ceramic, carbon, or sintered material on a high-power car. Give that choice real weight before it goes into a car that also sits in traffic.

Dual friction

A dual-friction disc puts a different facing on each side. The common street version puts Kevlar on the pressure-plate side and leaves the flywheel side organic. A street-and-strip version often pairs Kevlar with metal. Holding power stops at the weaker face. The strong side cannot carry torque the weak side has already given up.

Disc, Springs, and Clamp

The facing is only half the disc. How the material is cut, whether the hub has springs, and how hard the pressure plate clamps all change the car you drive every day.

Segmented and windowed discs are two ways of cutting the same heat path. The missing sections give the facing somewhere to dump temperature. A multi-disc, or multi-plate, clutch adds surfaces so the pack can hold more torque. The multi-plate clutches on the fastest Subarus are often treated as event parts: they do the weekend, then they come out.

Sprung hub or solid hub

The springs in a sprung hub move only a few millimeters. They are there to soak up the engine’s torsional pulses so the gears do not growl. The four noises a clutch maker tunes those springs for are acceleration noise, deceleration noise, coast rattle, and idle rattle. The springs are working once the disc is clamped. They are finished bottoming out long before they can soften a hard launch, so they do not cushion a drag-style engagement. Chatter from an aggressive facing is often worse with a sprung hub, because the springs are not a cure for a grabby material.

What the springs do reduce is vibration and shock that chew the disc splines and the gear teeth. A solid hub drops that damper. The disc is lighter, so shifts are easier on the synchros, and the gearbox gets noisier. Street cars want the sprung hub. A race disc that is shifted constantly is the usual place for a solid hub, with the noise accepted.

Clamp load

Pedal effort follows the pressure plate. An aftermarket clutch does not have to feel heavy. The increase over the stock pedal is whatever that plate was built to. More clamp does two other things. It raises the load on the crankshaft thrust bearings, and, together with a higher friction facing, it raises the shock that reaches the gears. The stock organic disc stays the most transmission-friendly clutch because the plate clamps lighter and the facing gives up grip when it is overheated.

The most torque comes from a high-clamp plate, a high-friction facing, and more surfaces, which is a twin-disc or triple-disc pack. That is also the combination with the heaviest side effects. Stage 1, Stage 2, and Stage 3 on a clutch box are marketing steps, the same way those words are marketing steps on a power package. Some makers use them and some do not. A staged clutch can still be the right part. The stage printed on the box is not a match for the stage of the tune.

Match the Clutch and Flywheel to the Gearbox

The flywheel follows the transmission. A five-speed gets a five-speed flywheel. A six-speed gets a six-speed flywheel. The engine bolted to that gearbox, including a swap, does not pick the wheel. These flywheels are neutral balance, with a symmetrical bolt pattern, so the wheel is not clocked to one hole the way an externally balanced engine’s flywheel is.

2002 spline count

A 2002 through 2005 WRX five-speed takes a 24-spline disc. The major diameter of that spline is 25.2 mm, and the clutch is the pull type. A 23-spline disc is the wrong part for that input shaft. The 2004 through 2008 STI six-speed clutch is also 24 spline at 25.2 mm. Spline count is not what separates the two discs. The flywheel and the fork are.

Early WRX flywheels

On a 2002 WRX the flywheel follows the build month, not the model year on the title. A wheel from a car built later in that run is a different part from a wheel built before June 2001. The disc, the pressure plate, and the throwout bearing are shared between the second and third wheels. The wheels themselves are not.

June 2001 is the changeover month, so a car built that month gets checked by its build date before the wheel is ordered. One 12342AA050 weighed 23.5 lb. A used wheel from “a 2002” can be the second part or the third, depending on whether that car was built before or after June 2002.

2011 five-speed flywheel

A 2011 WRX five-speed left the factory with a single-mass flywheel, part 12342AA090. The clutch is the push type. The friction face sits about 2 mm below the face the pressure plate bolts to. It is not a dual-mass wheel. On a dual-mass wheel the clutch face can turn a few degrees relative to the crank face, and those wheels are the Valeo units used on Legacy models. Hold the ring gear on a 12342AA090 and the friction face stays put. An organic disc built for a solid wheel belongs on this car. It does not belong on a Legacy dual-mass wheel.

A five-speed clutch can go into a six-speed if the six-speed clutch fork is the one on the car. A six-speed clutch can go into a five-speed, and that direction needs modification. Either swap is possible. A disc that happens to be on the shelf is a weak reason to do it.

On a stock or near-stock car, the organic disc is the part that fits the job. An aftermarket clutch holds more torque, wears out sooner, and hits the gearbox harder. People still buy one for a stock-power daily because they found a real bargain, or because they want the name. The car that makes stock torque is happier on the organic disc.

If the transmission is already out and the engine is about to make more torque, size the clutch for the torque that is coming and leave a margin, because a borderline rating is a slip waiting on the first hot pull. A daily that will still sit in traffic wants a sprung organic or Kevlar. A puck chosen only so the transmission never has to come out again makes every takeoff harsher than the torque required.

1. Torque first. Use flywheel torque, with margin over peak, and compare ratings inside one brand.
2. Daily or event. Sprung organic or Kevlar for the street. Puck, sintered, or multi-disc when the car can live with an on-off clutch.
3. One brand for the disc and plate. Then pick the flywheel for the five-speed or the six-speed.
4. Leave misfire detection on. A lighter flywheel can set a misfire light. Clearing the code is the safe response. Turning the test off hides a real miss.

A Lighter Flywheel

A lighter flywheel stores less energy, so the crank speeds up and slows down with less effort. On a dyno that usually shows little or no gain. In the car it feels large, because the gear multiplies the change. First gear has the most leverage, so the car feels quickest there. Each taller gear shrinks the effect, until vehicle weight dominates and the flywheel is a small piece of the job.

Pounds on a scale are a blunt way to compare wheels. Rotational inertia depends on where the mass sits. Ten pounds removed at the rim changes the wheel more than ten pounds removed near the hub. Two flywheels that weigh the same can feel different when the metal is piled at different radii. Holes out at the rim are one way makers chase that.

Low-speed manners are the compromise. Smooth starts need more rpm because there is less rotating mass to carry the car through the bite. A competitive drag launch wants that mass. Autocross, and a lot of daily driving, is where the quicker rev-matching pays off. Deceleration gear rattle is the noise almost everyone with a light flywheel hears. It can sound like a ring and pinion. It is usually the gears rattling with less inertia behind them, and it shows up on overrun, not as a new failure inside a freshly built gearbox.

The misfire light

There is no flywheel weight that is immune to a misfire light. The odds rise as the wheel gets lighter. A lighter crank pulley removes rotating mass as well, so the crank changes speed even faster, and the combination is more likely to set the code than the flywheel alone. A heavier light wheel can still set it. One car can run the same wheel clean while the next car lights the dash. Clearing the code and watching how often it returns is how you tell a light-flywheel code from a real miss.

What fits is a crank that speeds up and slows down faster than the calibration was built for, so the crank sensor and the cam sensor disagree on a tip-in or a closed-throttle decel. A belt tensioner that reacts slowly can add enough cam-belt slack to feed that disagreement. A WRX has one cam sensor and an STI has two, and STI cars throw this light less often. A given car can still go the other way.

The code from a light flywheel does not force limp mode. You can drive the car. Leaving the light on for weeks is still a bad habit, because a real fault can stack behind it and you will not see a second warning. Turning misfire detection off in the tune deletes the symptom and also deletes the warning for an actual miss.

Install and Break-In

Plan on about five hours when the tools and the experience are already there. This is a job to hand to a shop when they are not. The transmission has to come off, or the engine has to come forward. With a hoist, the engine comes out on two mounts and a pitch stop, and a timing belt, seals, and mounts can be done in the same opening. The service procedure unhooks the clutch fork from the throwout bearing before the cases separate. With the hinge pin already out, the fork can travel with the bearing, and some installs come apart that way.

A clutch kit is the disc, the pressure plate, the pilot bearing, and the release bearing. The flywheel is a separate part, sometimes sold in the same package. The fork is a steel lever and usually stays unless it is worn or bent. These turbo cars release the clutch with hydraulics. A cable on a generic parts list is the wrong part. A new release bearing often feels draggy. It arrives already packed with grease, and a smooth, consistent drag with no play is normal. Moly grease belongs on the input-shaft splines and on the contact pads of the fork.

Dowel pins are often missing from a new flywheel. A machine shop can pull them from the old wheel with a dowel-pin puller, which is a slide hammer with a fitting on the end, and the old pins drive into the new wheel. A drill press can walk an exposed pin out if the chuck is tight, because the pins are hard and a loose chuck just spins.

A stock Exedy flywheel is a special case. One owner who asked the manufacturer was told not to machine it after the first life, and to fit a new one for about another 90,000 miles. Do the clutch, the bearings, and the flywheel decision in one opening. The labor is the expensive half, and it does not get cheaper the second time.

The figures in that table are examples from 2005 and from later quotes across the years after that. A shop that does these transmissions every week tends to land at the low end of that labor spread. A shop seeing its first one tends to land at the high end, and the car is better off at the shop that has done it.

Break-in

A new clutch beds in the way new brake pads do. The window is generally under 1,000 miles, and the clutch maker’s own miles and procedure are the ones that count. Kevlar can glaze in traffic during that window and slip until the glaze wears off. Slip on the first boosted pull is not break-in. Oil from a rear-main seal that was seated wrong will glaze or contaminate a new disc the same way, and no amount of gentle miles cleans oil off the facing. A cooked organic disc can come back close to full grip. A cooked Kevlar disc does not.

Slip, Chatter, and Noise

Foot off the pedal, car in gear, engine pulling. Road speed and rpm move together on a clutch that is holding. If rpm climbs and the car does not, the disc is slipping. A loss of power with rpm and speed still locked together is somewhere else in the engine. Engagement that has moved high in the pedal travel is a worn disc, even before it slips. A hydraulic clutch does not get a cable adjustment to hide that.

Chatter on engagement with no slip can be a glazed disc or a flywheel face that is no longer flat. Hot spots that are only on the surface sometimes clean up, and sometimes the wheel is warped and the transmission has to come off again. A light chirp or a sproing with the pedal released is the classic fork-spring noise. It changes with weather and with what else is under the hood, and the car can be driven. A failed throwout bearing is the loud version of a pedal-out noise. You will not wonder which one it is. That bearing can also be driven for a while, and it is the part that is actually done.

A grinding noise that gets quiet when the clutch pedal goes down is in the gearbox, on the parts that stop turning when the input shaft unloads. A light flywheel makes deceleration rattle worse. It does not invent a bad gear set by itself. If the noise showed up the day after a different gear set went in, the new gears are the place to look, not the flywheel that was already on the car.

FAQ

Which clutch and flywheel should a daily WRX use?

A sprung organic disc and the stock flywheel, resurfaced on a Blanchard grinder or replaced if it is a stock Exedy wheel that has already been in service. Step up the facing only when flywheel torque is past what that organic disc can hold, and leave a margin.

Does the stage on the clutch match the stage on the tune?

No. Stage numbers on a clutch are a marketing ladder. Pick the disc from torque and from how the car is driven.

Can a five-speed clutch go in a six-speed?

Yes, with the six-speed clutch fork. A six-speed clutch in a five-speed needs modification. Either swap still has to be the right disc for the torque, not just a disc that bolts up.

Will a lighter flywheel add horsepower on a dyno?

Usually little or none. The change you feel is in the low gears, and it shrinks as the gear gets taller. Takeoff needs more rpm, and a misfire light gets more likely as the wheel gets lighter.

Should I turn the misfire light off in the tune?

No. The light from a light flywheel does not put the car in limp mode, and it can hide a real miss if you delete the test or ignore it for weeks. Clear it and see whether it comes back.

How do I know the clutch is slipping?

In gear, foot off the pedal, rpm and road speed move together when the disc is holding. Rpm that climbs while the car does not is slip. A high bite point is wear showing up before the slip.

Is a 2002 WRX clutch 23 spline or 24?

24 spline, 25.2 mm major diameter, pull type, for the 2002 through 2005 five-speed. A 23-spline disc does not fit that shaft.

Will a flywheel from before June 2001 fit a later 2002 WRX?

No. Before June 2001 is the first wheel. A car built in June 2001 is checked by its build date. From that changeover through May 2002 the wheel is 12342AA050, and from June 2002 it is 12342AA070. The disc, pressure plate, and throwout bearing are shared by the second and third wheels.

Did the 2011 WRX five-speed come with a dual-mass flywheel?

No. It came with single-mass part 12342AA090, a push-type wheel with the friction face about 2 mm below the pressure-plate face.

How long is clutch break-in?

Generally under 1,000 miles, on the schedule the clutch maker prints. Slip on the first hard pull is a problem, not part of bedding the disc in. Oil on the facing will not bed out.